Authorship：Lead author  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：CELL PRESS  

The long-lived U isotopes, U-233 and U-236, have been used increasingly in recent years as marine circulation tracers and for identifying sources of uranium contamination in the environment. The sedimentation histories of these two U isotopes in combination with natural U-238 were reconstructed for an anoxic sediment core collected from Beppu Bay, Japan, in the western North Pacific Ocean showing good time resolution (less than 2.6 y/sample). The U-233/U-236 atom ratio showed a prominent peak of (3.20 +/- 0.30) x 10(-2) around 1957 which can be attributed to the input from atmospheric nuclear weapons tests including thermonuclear tests conducting in the Equatorial Pacific. The integrated U-233/U-236 ratio of (1.64 +/- 0.08) x 10(-2) for the sediment was found to be in relatively good agreement with the representative ratio published for global fallout (similar to 1.4 x 10(-2)). A prominent increase in the authigenic ratio of U-233/U-238(a,s) in the leached fraction (1.39 +/- 0.11 x 10(-11)) and the bulk digestion (1.36 +/- 0.10 x 10(-11)) was also observed around 1957. This reflects the input supply of U-233 to the seawater which is known to have a relatively constant U-238 content. The authigenic U-236/(238)Ua,s ratio (0.18 +/- 0.02 x 10(-9)) obtained for 1921 increased from the early 1950's to a maximum of (6.59 +/- 0.60) x 10(-9) around 1962. The variation in this ratio represents well the introduction history of U into the surface environment without site-specific U contamination and the time profile is also consistent with the Cs-137 signature. This work thus provides a benchmark for the long-term use of the isotopic U composition as an input parameter for seawater circulation tracers and as a chronological marker for anoxic sediments and sedimentary rocks. Especially the U-233/U-236 ratio may serve as a keymarker for the new geological age Anthropocene.

DOI： 10.1016/j.heliyon.2023.e14153

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY  

Copepods play a key trophic role as secondary producers, transferring primary production to higher trophic levels such as fish. Copepod production contributes significantly to successful fish recruitment. Despite their importance, knowledge of copepod dynamics over several decades remains limited due to the lack of long-term data series with adequate sampling and analysis. However, an understanding of long-term copepod dynamics is urgently required to strive toward better management for sustainable aquatic ecosystems and fish recruitment. Sedimentary DNA (sedDNA) has been developing as a useful tool for reconstructing past plankton dynamics. This study evaluates whether sedDNA targeting the pelagic copepod, Eodiaptomus japonicus, in Lake Biwa (Japan) can be an effective tool for elucidating its past population dynamics. We applied a quantitative polymerase chain reaction method targeting the mitochondrial cytochrome c oxidase subunit I gene on two sediment cores and compared the detected sedDNA concentrations with the unique long-term dataset of demographic traits, biomass, specific growth rate, production, subitaneous eggs, and resting eggs of E. japonicus. The sedDNA concentration of E. japonicus recovered from sediment layers correlated significantly with in situ production, biomass, and production of immediately hatching eggs but not with resting eggs or specific growth rate. Our study provides evidence for the effective use of sedDNA as a tracking tool for assessing past copepod production dynamics.

DOI： 10.1111/fwb.14096

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.scitotenv.2022.158751

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SAGE Publications  

For assessment of the potential of the Beppu Bay sediments as a Global Boundaries Stratotype Section and Point (GSSP) candidate for the Anthropocene, we have integrated datasets of 99 proxies. The datasets for the sequences date back 100 years for most proxy records and 1300 years for several records. The cumulative number of occurrences of the anthropogenic fingerprint reveal unprecedented increases above the base of the 1953 flood layer at 64.6 cm (1953 CE), which coincides with an initial increase in global fallout of ²³⁹Pu+²⁴⁰Pu. The onset of the proliferation of anthropogenic fingerprints was followed by diverse human-associated events, including a rapid increase in percent modern ¹⁴C in anchovy scales, changes in nitrogen and carbon cycling as recorded by anchovy δ¹⁵N and δ¹³C, elevated pollution of heavy metals, increased depositions of novel materials (spheroidal carbonaceous particles, microplastics, polychlorinated biphenyls), the occurrence of hypoxia (Re/Mo ratio) and eutrophication (biogenic opal, TOC, TN, diatoms, chlorophyll a), unprecedented microplankton community changes (compositions of carotenoids, diatoms, dinoflagellates), abnormally high spring air temperatures as inferred from diatom fossils, and lithological changes. These lines of evidence indicate that the base of the 1953 layer is the best GSSP level candidate in the stratigraphy at this site.

File： Kuwae 2023 Beppu Bay, Japan, as a candidate Global boundary Stratotype Section and Point for an Anthropocene series.pdf

DOI： 10.1177/20530196221135077

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Other Link： http://journals.sagepub.com/doi/full-xml/10.1177/20530196221135077

Authorship：Last author   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

<title>Abstract</title>Environmental DNA (eDNA) is currently developing as a powerful tool for assessing aquatic species dynamics. However, its utility as an assessment tool for quantification remain under debate as the sources of eDNA for different species is not always known. Therefore, accumulating information about eDNA sources from different species is urgently required. The objective of our study was to evaluate whether sedimentary DNA targeting two <italic>Daphnia</italic> species, <italic>D. galeata</italic> and <italic>D. pulicaria</italic>, could track <italic>Daphnia</italic> population dynamics and resting egg production. Applying a quantitative PCR targeting the mitochondrial 12S rRNA gene on sediment cores collected in Lake Biwa, Japan, we compared sedimentary DNA concentration of <italic>Daphnia</italic> with the abundance of remains and ephippia, reflecting their abundance and resting egg production, respectively. We found that the sedimentary DNA concentrations of <italic>Daphnia</italic> for the past century were inconsistent with their population abundance. However, the concentration was highly correlated with the resting egg production. Our results provide evidence that ephippia with resting eggs, released during spawning activities, was a significant source of <italic>Daphnia</italic> DNA archived in sediments. Our work provides critical insights for using sedimentary DNA as a monitoring tool for egg production dating back 100 years.

DOI： 10.1038/s41598-021-03899-0

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Other Link： https://www.nature.com/articles/s41598-021-03899-0

Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

Plutonium (Pu) has been used as a mid-twentieth century time-marker in various geological archives as a result of atmospheric nuclear tests mainly conducted in 1950s. Advancement of analytical techniques allows us to measure ²³⁹Pu and ²⁴⁰Pu more accurately and can thereby reconstruct the Pacific Pu signal that originated from the former Pacific Proving Grounds (PPG) in the Marshall Islands. Here, we propose a novel method that couples annual banded reef building corals and nearshore anoxic marine sediments to provide a marker to precisely determine the start of the nuclear era which is known as a part of the Anthropocene. We demonstrate the efficacy of the methods using sediment obtained from Beppu Bay, Japan, and a coral from Ishigaki Island, Japan. The sedimentary records show a clear Pu increase from 1950, peaking during the 1960s, and then showing a sharp decline during the 1970s. However, a constantly higher isotope ratio between ²³⁹Pu and ²⁴⁰Pu suggest an additional contribution other than global fallout via ocean currents. Furthermore, single elevations in ²⁴⁰Pu/²³⁹Pu provide supportive evidence of close-in-fallout similar to previous studies. Coral skeletal radiocarbon displays a clear timing with the signatures supporting the reliability of the Beppu Bay sediments as archives and demonstrates the strength of this method to capture potential Anthropocene signatures.

DOI： 10.1038/s41598-022-14179-w

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Other Link： https://www.nature.com/articles/s41598-022-14179-w

Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1007/s10750-022-05071-9

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Other Link： https://link.springer.com/article/10.1007/s10750-022-05071-9/fulltext.html

Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

DOI： 10.1029/2022jg006825

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SAGE Publications  

Spheroidal carbonaceous particles (SCPs) are carbonaceous fly ash particles produced solely from industrial fossil fuel combustion. SCPs in sediments can be an indicator of past industrialization. We examined the sedimentary SCP record in Beppu Bay, southern Japan, and compared this record to historical trends of industrial activity and monitoring data for atmospheric pollutions in the region. Beppu Bay has varved sediments in the absence of bioturbation, providing an ideal situation for SCP study. Our results show that the temporal variation of SCP deposition is consistent with the trends of industrial activity and the temporal variation of status of atmospheric pollution. We conclude that undisturbed sediments such as Beppu Bay sediments have SCP records that adequately represent historical trends of industrial activity and atmospheric pollution in detail. These findings support that SCPs are a potential marker for the Anthropocene, characterized by industrialization.

DOI： 10.1177/20530196221076577

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Other Link： http://journals.sagepub.com/doi/full-xml/10.1177/20530196221076577

Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.ancene.2021.100318

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：FRONTIERS MEDIA SA  

Stratigraphic cluster analysis using aquatic palynomorphs preserved in the core sediments revealed a rapid eutrophication due to anthropogenic activities from the mid 1960s in Beppu Bay, East Kyushu, Japan. These assemblages were divided into three major units: BP-I, BP-II and BP-III, and also only dinoflagellate cyst assemblages were divided into the following four units in Beppu Bay: BP-A, BP-B, BP-C, and BP-D. Unit boundaries based on aquatic palynomorphs and dinoflagellate cysts were different except in the upper part, BP-III and BP-D, both of which clearly indicated anthropogenic eutrophication in both sea water and bottom sediments. On the other hand, in dinoflagellate cyst assemblages, Unit BP-A was characterized by stable occurrence of Spiniferites bulloideus and Spiniferites hyperacanthus, Lingulodinium machaerophorum of Gonyaulacales, and reduction of heterotrophic Peridinioid Brigantedinium spp. In Unit BP-C there was a clear decrease of L. machaerophorum. Unit BP-B was characterized by decreases of S. bulloideus, S. hyperacanthus, and L. machaerophorum, and little increase of Spniferites bentori. Unit BP-C was characterized by an increase in S. bulloideus and heterotrophic Peridinioid Echinidinium spp. Unit BP-D was subdivided into Subunit BP-D1 where dinoflagellate cysts showed a marked increase in S. bulloideus accompanied by the appearance of L. machaerophorum and Tuberculodinium vancampoae, and Subunit PB-D2 where there was a decrease of total dinoflagellate cysts. From the dinoflagellate cyst assemblages, the marine environment of the period of BP-A Unit was suggested to be warm and stable. However, L. machaerophorum started to decrease in BP-B. The clear decrease of L. machaerophorum suggest that the marine environment became cooler than that of Unit BP-A. Significant increases of S. bulloideus, S. hypearcanthus, L. machaerophorum, T. vancampoae, Brigantedinium spp., and Polykrikos kofoidii were characteristic of Unit BP-D. The increase in total dinoflagellate cyst density and the increase of the ratio of heterotrophic dinoflagellate cysts in Subunit BP-D1 are manifestations of the Oslo fjord Signal and Heterotroph Signal, respectively. In addition, the decrease in microforaminiferal lining that continued from Unit BP-C to Unit BP-D might indicate deterioration of the bottom sediment environment.

DOI： 10.3389/fmars.2022.843824

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Cambridge University Press (CUP)  

Abstract

Little is known about the dynamics of marine food chains spanning primary to higher trophic levels on centennial and longer timescales, especially where the supply of dissolved iron limits primary productivity. To elucidate the long-term dynamics of biological productivity in the Coastal Oyashio (CO), which is a major pathway for transporting dissolved iron into the western North Pacific from winter to spring, we reconstructed the lower trophic level productivity over the last 3000 years in the CO. Our results demonstrate that the concentrations and mass accumulation rates of both Chl-a (chlorophyll a and its derivatives) and biogenic opal used as proxies of primary productivity, and steryl chlorin esters (SCEs) used as that of zooplankton productivity, show a millennial-scale increasing trend and centennial-scale variability beginning ca. AD 400. SCEs were positively correlated with Chl-a, indicating that changes in zooplankton productivity were induced by bottom-up control of primary productivity. The Chl-a and SCEs showed synchronous centennial-scale patterns with a relative abundance of sea-ice-associated diatom species transported by CO, and with a ventilation index in the Okhotsk Sea Intermediate Water. This synchronous pattern indicates that lower trophic-level productivity during the spring bloom responded to the intensity of iron-replete CO.

File： Kuwae 2022 Late Holocene centennial to millennial-scale variability in lower trophic level productivity off southern Hokkaido, Japan and its response to dissolved iron-replete Coastal Oyashio dynamics..pdf

DOI： 10.1017/qua.2021.71

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

The development of rapid and efficient analytical method for the determination of legacy and current-use brominated flame retardants (BFRs) has been performed due to environmental concern related to these pollutants. In the present study, we used an automated clean-up device equipped with pre-packed micro-column sets (containing sulfuric acid impregnated silica gel and silver-modified alumina) to develop an effective purification method for polybrominated diphenyl ethers (PBDEs), pentabromoethylbenzene, hexabromobiphenyl, and decabromodiphenyl ethane (DBDPE) in sediment extracts. Matrix-spiked sediments (n = 6) and the Standard Reference Material (R) 1944 samples (n = 6) were tested. Our method showed acceptable accuracy, repeatability, and sensitivity for almost all the target compounds with reduced processing time, labor requirement, and solvent amounts as compared to conventional clean-up method (e.g., sulfuric acid treatment and self-packed chromatographic columns). The validated method was applied to sediment core samples (n = 16) collected in 2019 from Lake Biwa, the largest lake in Japan. PBDEs were detected in sediment samples of 0-13 cm depth (dated between 1990 and 2019) at relatively low concentrations (median 5.7; range 2.6-9.4 ng/g dry weight). PBDE profiles were dominated by BDE-209, which accounted for 91 +/- 10% of total PBDEs. Among other BFRs, only DBDPE was found in sediment layers of 0-9 cm depth (deposited between 2005 and 2019). DBDPE concentrations ranged from 0.43 to 1.6 (median 0.71) ng/g and showed increasing trend toward shallower depths.

DOI： 10.1016/j.chemosphere.2021.130867

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

The occurrence of 209 PCB congeners was determined in a sediment core dated between 1930 and 2019 from Lake Biwa, a typical temperate monomictic lake in Japan. Concentrations of total PCBs ranged from 5.3 to 48 ng/g dry weight (dw), showing a highest peak at the 1960s to 1970s. The temporal trend of total PCBs in this sediment core generally matched with Japanese PCB production and emission pattern (i.e., increasing from the 1950s, peaking at 1970, and gradually decreasing since 1972). The vertical PCB profiles in our core were affected by physical mixing and bioturbation. By using a detailed and comprehensive analytical method, we have found elevated concentrations and special historical profiles of several congeners such as CB-7,-11,-47/48/75,-51,-68, and-209, which are still rarely included in routine PCB analysis. Some tetra-CB congeners like CB-47/48/75,-51, and-68 showed their concentration peaks at the early 2010s, which may be unintentionally produced during polymer manufacturing processes. PCB homolog-and congener-specific profiles in our sediment core samples have experienced weathering with higher proportions of penta-and hexa-CBs as compared to the Kanechlor usage pattern (i.e., dominated by tri-and tetra-CBs). Both intentional (i.e., technical mixtures) and unintentional (e.g., PCB-containing polymers and pigments) sources of PCBs were suggested from congener specific analysis.(c) 2021 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.scitotenv.2021.147913

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1007/s10201-020-00646-8

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Other Link： http://link.springer.com/article/10.1007/s10201-020-00646-8/fulltext.html

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.35248/2375-4397.21.9.283

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

In recent years, concern about the release of anthropogenic organic micropollutants referred to as contaminants of emerging concern (CECs) has been growing. The objective of this study was to find potential CECs by means of an analytical screening method referred to as comprehensive target analysis with an automated identification and quantification system (CTA-AIQS), which uses gas and liquid chromatography combined with mass spectrometry (GC-MS and LC-QTOF-MS). We used CTA-AIQS to analyze samples from a sediment core collected in Beppu Bay, Japan. With this method, we detected 80 compounds in the samples and CTA-AIQA could work to useful tool to find CECs in environmental media. Among the detected chemicals, three PAHs (anthracene, chrysene, and fluoranthene) and tris(isopropylphenyl)phosphate (TIPPP) isomers were found to increase in concentration with decreasing sediment depth. We quantified TIPPP isomers in the samples by means of targeted analysis using LC-MS/MS for confirmation. The concentration profiles, combined with previous reports indicating persistent, bioaccumulative, and toxic properties, suggest that these chemicals can be categorized as potential CECs in marine environments.

DOI： 10.1016/j.envpol.2020.115587

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Contamination levels and temporal trends of polybrominated diphenyl ethers (PBDEs) and some alternative brominated flame retardants (BFRs) were examined in a dated sediment core from the deepest part of the Beppu Bay, southwestern Japan. PBDEs were found in the upper layers of 0-15 cm depth at concentrations ranging from 5200 to 32,600 pg g-1 with the peak estimated at 1995. Decabromodiphenyl ether (BDE-209) was the most abundant congener, accounting for 96% in average of total PBDEs. The vertical profile of BDE-209 observed in our sediment core generally agreed with the historical pattern of domestic demand of commercial deca-BDE mixtures in Japan, and perfectly matched with maximum stock of these products (i.e., 42,000 tons in 1995). Among alternative BFRs, only decabromodiphenyl ethane (DBDPE), a replacement of deca-BDE, was found at significant levels with concentrations of 69-850 pg g-1 in sediment layers dated between 1991 and 2011. Ratios of DBDPE to BDE-209 gradually increased during this period, implying opposite trends of these two compounds and the role of DBDPE as a deca-BDE's alternative. The occurrence of deca-BDE components in sediments may pose medium risk to benthic aquatic life, while the ecological risk of other PBDE homologs and DBDPE was negligible.

DOI： 10.1016/j.chemosphere.2020.129180

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PubMed

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

<title>Abstract</title>
Far too little is known about the long-term dynamics of populations for almost all macro-organisms. Here, we examined the utility of sedimentary DNA techniques to reconstruct the dynamics in the “abundance” of a species, which has not been previously defined. We used fish DNA in marine sediments and examined whether it could be used to track the past dynamics of pelagic fish abundance in marine waters. Quantitative PCR for sedimentary DNA was applied on sediment-core samples collected from anoxic bottom sediments in Beppu Bay, Japan. The DNA of three dominant fish species (anchovy, sardine, and jack mackerel) were quantified in sediment sequences spanning the last 300 years. Temporal changes in fish DNA concentrations are consistent with those of landings in Japan for all three species and with those of sardine fish scale concentrations. Thus, sedimentary DNA could be used to track decadal-centennial dynamics of fish abundance in marine waters.

File： Kuwae 2020 Sedimentary DNA tracks decadal-centennial fish abundance.pdf

DOI： 10.1038/s42003-020-01282-9

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Other Link： http://www.nature.com/articles/s42003-020-01282-9

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

Contamination levels and profiles of mono- to deca-chlorinated biphenyls (PCBs) were characterized in a sediment core dated in 1954-2011 from Beppu Bay, southwestern Japan, providing a comprehensive and detailed picture on the environmental occurrence, temporal trends, and emission sources of these pollutants in the study area. Concentrations of total PCBs in the core ranged from 3.5 to 150 (median 15) ng g ' dry weight and exhibited depth profile matching with Japanese PCB production and emission patterns (i.e., drastically increasing from the early 1960s, peaking in 1970, and then rapidly decreasing). Origin of PCBs in the studied samples largely associated with Kanechlor mixtures (e.g., KC-300 and KC-400), especially for sediment layers dated between the mid-1960s and early 1970s (i.e., the intensive PCB production period in Japan). In addition, dechlorination and weathering signals and emerging inputs of PCBs were also observed in deeper and shallower sediment segments with notable proportions of some unique congeners such as CB-47/48/51 and CB-11. respectively. Historical fluxes of PCBs in our samples showed quite similar vertical shape as concentrations. In the context of national implementation for complete treatrnent of PCB-containing waste until 2024, further investigations on spatiotemporal trends and environmental loads of PCBs in japan are necessary. (C) 2020 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.scitotenv.2020.140767

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

Egg dormancy is a key life history strategy in copepods to temporarily escape catastrophic or repetitive detrimental events and ensure permanent colonization. In Lake Biwa, although the copepod Eodiaptomus japonicus dominates the zooplankton community year-round, it has been known to produce resting eggs, yet little is known about its dormancy. To clarify the nature of dormancy in E. japonicus, we collected a sediment core from Lake Biwa, isolated resting eggs, and conducted hatching experiments to determine the life-cycle traits of the hatched individuals. We isolated 242 eggs (maximum age, 60 years) from the upper 17 cm depth sediments, and although no strict catastrophic event such as seasonal drying up was found, the findings implied that E. japonicus might have a "bet-hedging" strategy. Accumulated egg density above 17 cm was 2.5 x 10(4) m(-2), and mean annual egg flux was 10(3) m(-2) per year. Over the first 10 cm sediments, average egg hatching success was 17%. No egg hatchings were confirmed below 10 cm depth. Embryonic development times of the resting eggs were longer than those of subitaneous eggs, indicating that a lag phase existed until development resumed; 33% of hatched nauplii molted to adulthood, and 80% of females produced offspring. These results suggest potential recruitment from resting eggs in the bottom sediments, which partly serve as an egg bank in this copepod.

DOI： 10.1080/20442041.2019.1671766

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Paleorecords of pelagic fish abundance could better define the nature of fishery productivity dynamics and help understand responses of pelagic fish stocks to long-term climate changes. We report a high-resolution record of sardine and anchovy scale deposition rates (SDRs) from Beppu Bay, Southwest Japan, showing multidecadal and centennial variability in the abundance of Japanese sardine and Japanese anchovy during the last 2850 years. Variations in the sardine SDR showed periodicities at similar to 50, similar to 100, and similar to 300 yr, while variations in the anchovy SDR showed periodicities at similar to 30 and similar to 260 yr. Comparisons between and correlation analyses of the time series of the sardine and anchovy SDRs demonstrate that there is not a consistent out-of-phase relationship during the last 2850 years. This indicates that the multidecadal alternations in the sardine and anchovy populations commonly seen in the 20th century did not necessarily occur during earlier periods. The Japanese sardine SDR record shows a long-term decreasing trend in the amplitudes of the multidecadal to centennial fluctuations. This decreasing trend may have resulted from an increasing trend in the winter sea surface temperature in the western North Pacific. The multicentennial variability in sardine abundance during the last millennium is consistent with the variabilities in the abnormal snow index in East Asia and the American tree ring-based Pacific Decadal Oscillation index, suggesting a basin-wide or regional climate-marine ecosystem linkage.

DOI： 10.1016/j.pocean.2017.09.011

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

In this study, we revised eruptive ages, especially the Aso and Yufu volcanoes, central Kyushu, by analyzing a sediment core of southwestern Beppu Bay, southwest Japan, where undisturbed sediments are continuously deposited in a dysoxic basin. We identified two ash-fall layers in the sequence covering the last ca. 3000 years by using the refractive index of volcanic glasses and other parameters. These layers were likely correlated with N2 ash of the Aso volcano and Yufu-dake 1 ash (Yf1) of the Yufu volcano on the basis of the refractive indices of volcanic glasses, heavy mineral compositions, special distribution, and facies. According to high precision age models, the eruptive ages (68.2% probability range) of these tephras were 1470-1490 and 2010-2100 cal yr BP, respectively, and concordant with reported ages of previous research. These precise ages of tephras improve linking or dating in the various environments, and contribute to investigation of environmental changes or sedimentary process from source to sink. (C) 2017 Elsevier Ltd and INQUA. All rights reserved.

DOI： 10.1016/j.quaint.2017.01.024

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

We examined fossil pigments in a Pb-210-dated sediment core to document the temporal variations in phytoplankton biomass over the past 150 years in a semi-enclosed bay, Beppu Bay, in the western Seto Inland Sea, Japan. The flux of fossil pigments was used as an index of phytoplankton biomass, which we reconstructed after removing the effect of post-burial degradation on the concentrations of fossil pigments. The flux doubled from the 1960s to the early 1970s, decreased or remained stable in the early 1980s, and increased again from the late 1980s to the early 1990s. The first increase in phytoplankton biomass during the 1960s was likely caused by eutrophication due to an increase in terrestrial nutrient fluxes from watersheds. The decreasing phytoplankton biomass in the early 1980s was likely related to the establishment of a sewage treatment system that reduced the terrestrial nutrient fluxes to the sea. However, the terrestrial nutrient fluxes could not explain the second increase from the late 1980s to the early 1990s. Intensification of the influx of nutrients from the shelf slope to the sea was likely the cause of the second increase in phytoplankton biomass. This is supported by the inverse relationship between phytoplankton biomass and sea level at the shelf slope, the latter being an index of the intensity of the influx of oceanic nutrients from the shelf slope to the sea. The supply of oceanic nutrients may be therefore a critical factor in the determination of primary production in the western Seto Inland Sea.

DOI： 10.1007/s10872-016-0404-y

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Lignins have been used as a biomarker to explore changes in terrestrial organic matter input into lakes and to investigate past watershed vegetation. Burial of organic carbon (OC) in lake sediments, an important component of the global C cycle, is likely associated with the terrestrial OC input. However, few studies have explored changes in terrestrial C input into lakes in the last century. Furthermore, the relationship between lignin phenol compositions and watershed vegetation remains poorly examined. In this study, we examined changes in OC concentrations, OC mass accumulation rates (MAR), and lignin phenol compositions over the last century in sediments from six lakes in Japan that differ in watershed land-use and vegetation. The sediments were dated using Pb-210 and Cs-137, and showed increased OC concentrations and MARs in three lowland lakes over the last century. This pattern was not found in three mountain lakes. In one of the lowland lakes, lignin phenol concentrations normalized to OC did not change during the periods with high OC concentrations and MARs. This indicates that not only eutrophication but also enhanced terrestrial OC input could lead to greater burial of OC. The lignin phenol compositions did not show clear trends over the last century in most of the lakes examined. The ratios of syringyl to vanillyl phenols and the lignin phenol vegetation index had significant relationships with proportions of angiosperms in watershed vegetation. These results demonstrate that lignin phenols are useful in inferring recent as well as past changes in lake watershed environments. (c) 2016 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.orggeochem.2016.11.001

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Sediment cores from six lakes situated from north to south on the Japanese Archipelago were collected during 2009-2010 to investigate the hypothesis that deposition of lead (Pb) was coming from East Asia (including China, South Korea and eastern part of Russia). Accumulation rates and ages of the lake sediment were estimated by the Pb-210 constant rate of supply model and Cs-137 inputs to reconstruct the historical trends of Pb accumulation. Cores from four lakes located in the north and central Japan, showed clear evidence of Pb pollution with a change in the Pb-206/Pb-207 and Pb-208/Pb-207 ratios in the recent sediment as compared to the deeper sediment. Among the six studied lakes, significant inputs of anthropogenic lead emissions were observed at Lake Mikazuki (north Hokkaido in north Japan), Lake Chokai (north of Honshu), and Lake Mikuriga (central part of Honshu). Pb isotopic comparison of collected core sediment and previously reported data for wet precipitation and aerosols from different Asian regions indicate that, before 1900, Pb accumulated in these three lakes was not affected by trans-boundary sources. Lake Mikazuki started to receive Pb emissions from Russia in early 1900s, and during the last two decades, this lake has been affected by trans-boundary Pb pollution from northern China. Lake Chokai has received Pb pollutant from northern China since early 1900s until 2009, whereas for the Lake Mikuriga the major Pb contaminant was transported from southern China during the past 100 years. The results of our study demonstrate that Japan Archipelago has received trans-boundary Pb emissions from different parts of East Asian region depending on location, and the major source region has changed historically. (C) 2016 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.scitotenv.2016.03.138

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Strike-slip basins are one of the most important accumulation spaces for sediment of terrigenous, biogenic, and volcanic origins, and generally include large amount of event deposits. Although these event deposits are important basin filling process, research on this topic, particularly the effects of event deposits, is insufficient. In this study, we discuss sedimentation features based on grain composition and other properties for ca. 3000 year periods in Beppu Bay, which is strike-slip basin located at the western end of an arc-bisecting dextral fault known as Median Tectonic Line (MTL) associated with the northwestward subduction of the Philippine Sea Plate. This sediment is composed of hemipelagic clay and coarser event layers of turbidites referred to as types A, B, and C; ash layers referred to as type D; and other referred to as type E. The turbidite event layers, which accounted for 92% of the total major event layer, with &gt;1 cm thickness, consist of particles related to volcanism, including hydrothermal activity. The events control the regional filling rate and transportation of coarse and heavy volcaniclastic materials. In particular, type A, which accounted for 73% of the total major event layer thickness, is likely induced by earthquakes related to the MTL, according to its age. As a result, the basin filling processes are controlled mainly by tectonics related to the subduction of the Philippine Sea Plate. (C) 2015 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jseaes.2015.12.008

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Centennial-scale variability of the East Asian winter monsoon during the Holocene is poorly understood because suitable archives and proxies are lacking. Here we present a high-resolution (similar to 30-yr spacing) planktonic foraminiferal delta O-18 record of Neogloboquadrina incompta (dextral form), which reflects sea surface temperature during the winter season, for the last 6000 yrs from marine sediments in the western North Pacific. Stronger winter monsoons indicated by cooler winter SSTs correspond to weaker summer monsoons indicated by the cave oxygen isotopes in centennial-scale variability. The variability also shows good correlation with delta O-18 records in lake sediments and ice cores from the Yukon Territory, Canada, spanning the last 4500 yrs, suggesting east-west climate coupling across the North Pacific. Furthermore, the climate changes across the North Pacific co-vary over widespread regions, such as the eastern tropical Pacific and the northern Red Sea, and the reconstructed solar activity. The cross-spectral and wavelet analyses show that the East Asian winter monsoon shares some cyclicity with the solar variability. Our results suggest that the solar activity is a fundamental forcing producing the centennial-scale EAWM variability mediated by the large-scale climate linkages. (C) 2014 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.epsl.2014.03.043

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PUBLIC LIBRARY SCIENCE  

Phospholipid-derived fatty acids (PLFA) and respiratory quinones (RQ) are microbial compounds that have been utilized as biomarkers to quantify bacterial biomass and to characterize microbial community structure in sediments, waters, and soils. While PLFAs have been widely used as quantitative bacterial biomarkers in marine sediments, applications of quinone analysis in marine sediments are very limited. In this study, we investigated the relation between both groups of bacterial biomarkers in a broad range of marine sediments from the intertidal zone to the deep sea. We found a good log-log correlation between concentrations of bacterial PLFA and RQ over several orders of magnitude. This relationship is probably due to metabolic variation in quinone concentrations in bacterial cells in different environments, whereas PLFA concentrations are relatively stable under different conditions. We also found a good agreement in the community structure classifications based on the bacterial PLFAs and RQs. These results strengthen the application of both compounds as quantitative bacterial biomarkers. Moreover, the bacterial PLFA- and RQ profiles revealed a comparable dissimilarity pattern of the sampled sediments, but with a higher level of dissimilarity for the RQs. This means that the quinone method has a higher resolution for resolving differences in bacterial community composition. Combining PLFA and quinone analysis as a complementary method is a good strategy to yield higher resolving power in bacterial community structure.

DOI： 10.1371/journal.pone.0096219

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We analyzed the lithology, magnetic susceptibility, bulk density, and X-ray radiographs of 14 sediment cores (1-9m long) from Beppu Bay in the western Seto Inland Sea, Japan, to establish the late Holocene stratigraphy in the deepest part of the bay and to develop an age-depth model for the sediments there. The cores contained 18 thick (major event) high-density layers (16 turbidites and two volcanic ash
&gt
1cm thick), and both lithological observations and density variations in the hemipelagic mud that is dominant in the cores revealed a further 55 thin (minor event) high-density layers (&lt
1cm thick). Analyses of color properties and opal and sand contents of the hemipelagic mud defined nine lithological units. After stratigraphic correlation of the event layers among cores, we projected 14C dates onto a single composite core. Forty-two AMS 14C dates from bivalve mollusk shells were used to construct a wiggle-matching-based age-depth model for the late Holocene sequence and to determine the local reservoir effect (ΔR). The age-depth model showed a sedimentation rate of 0.23-0.30cm/yr for a 7.8m-long composite core and an age of ∼2800calyr BP at the base. Wiggle-matching provided ΔR values of 115-155yr for late Holocene bivalve samples from Beppu Bay, which is consistent with previous estimates reported from coastal areas near the Kuroshio Front. Comparison of wiggle-matching-derived ages of thick turbidites with the ages of historical earthquakes showed differences within ±25yr. Our study demonstrated that wiggle matching with optimal fitting based on either the weighted least-squares or maximum likelihood method can minimize the effect of scatter of age data due to reworking and burrowing of bivalves and thus improve the accuracy of age-depth models. © 2012 Elsevier Ltd.

DOI： 10.1016/j.jseaes.2012.07.002

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

The oxygen isotopic composition (δ18O) of planktonic foraminiferal shells in seafloor sediment provides information on past surface oceanography. Knowledge of seasonal and depth habitat, as well as the δ18O disequilibrium (vital effect), is essential to constrain the interpretation of sedimentary δ18O. Here, we present a 1-year time series of planktonic foraminiferal shell fluxes and δ18O from a sediment trap moored in the northwestern margin of the North Pacific. The vital effect and calcification depth for four species were estimated by comparing shell δ18O and the predicted values of equilibrium calcite calculated from temperature and estimated δ18O in seawater. Six major species (Neogloboquadrina incompta, Neogloboquadrina dutertrei, Neogloboquadrina pachyderma, Globigerina quinqueloba, Globigerina bulloides, and Globorotalia scitula) constituted 97% of the total foraminiferal flux. Most major species showed large fluxes in June and December, corresponding to periods of the development and disruption of the seasonal thermocline, implying the importance of nutrient injection and/or circulation for foraminiferal fluxes. Additional peaks in N. dutertrei and N. pachyderma were observed in August. The seasonal successions of foraminiferal fluxes corresponded to surface ocean stratification conditions and food availability, which are closely related to circulation of local currents. Vital effect estimations suggest that shells calcified in equilibrium for G. bulloides and N. pachyderma [sinistral (s)] and with a -0.7‰ offset for N. dutertrei [dextral (d)], a -1.0‰ offset for N.incompta (d), and a -0.3‰ offset for N. pachyderma (d). The calculation of flux-weighted δ18O values reveals that the sedimentary δ18O values of G. bulloides, N. dutertrei (d), and N. incompta (d) reflect surface temperature in winter season, and those of N. pachyderma (s) and N. pachyderma (d) reflect summer and annual mean subsurface temperature, respectively. The shallow calcification depths for the four species suggest that δ18O between different species (δδ18O) in the western North Pacific does not work for reconstructing past stratification conditions, unlike in other regions. Rather, the δδ18O between N. pachyderma (s) and G. bulloides, N. dutertrei (d) or N. incompta (d) may be a more suitable proxy for past seasonality. © 2013 Elsevier B.V.

DOI： 10.1016/j.marmicro.2013.03.013

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Concentrations of 18 elements, including Sb, In, Sn, and Bi, were measured in sediment cores from two pristine alpine lakes on Mount Hachimantai, northern Japan, representing the past 250years. Vertical variations in concentrations are better explained by atmospheric metal deposition than by diagenetic redistribution of Fe and Mn hydroxide and organic matter. Anthropogenic metal fluxes were estimated from 210Pb-derived accumulation rates and metal concentrations in excess of the Al-normalized mean background concentration before 1850. Anthropogenic fluxes of Sb and In showed gradual increases starting around 1900 in both lakes, and marked increases after 1980. Comparison of Sb/Pb and Pb stable isotope ratios in sediments with those in aerosols of China or northern Japan and Japanese source materials (recent traffic- and incinerator-derived dust) suggest that the markedly elevated Sb flux after 1980 resulted primarily from enhanced long-range transport in aerosols containing Sb and Pb from coal combustion on the Asian continent. The fluxes of In, Sn, and Bi which are present in Chinese coal showed increasing trends similar to Sb for both study lakes. This suggests that the same source although incinerators in Japan may not be ruled out as sources of In. The sedimentary records for the last 250years indicate that atmospheric pollution of Sb and In in East Asia have intensified during recent decades. © 2012 Elsevier B.V.

DOI： 10.1016/j.scitotenv.2012.10.037

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER JAPAN KK  

Atmospheric dust has wide-reaching effects, not only influencing climate conditions, but also ecosystems. The eastern region of the Asian continent is one of the largest emitters of dust in the world, and recent economic growth in the region has been accompanied by an increase in anthropogenic emissions. However, the effects of increased Asian dusts on aquatic ecosystems are not well understood. We examined fossil pigments and zooplankton remains from Pb-210-dated sediments taken from high mountain lakes of Hourai-Numa and Hachiman-Numa, located in the Towada-Hachimantai National Park of Japan Islands, to uncover historical changes in the phyto-and zooplankton community over the past 100 years. Simultaneously, we measured the geochemical variables of TOC, TN, TP, delta C-13, delta N-15, and lead isotopes (Pb-207/Pb-206, Pb-208/Pb-206) in the sediments to identify environmental factors causing such changes. As a result, despite few anthropogenic activities in the watersheds, alpine lakes in Japan had increased algal and herbivore plankton biomasses by 3-6 fold for recent years depending on the surrounding terrestrial vegetation and landscape conditions. Biological and biogeochemical proxies recorded from the lake sediments showed that this eutrophication occurred after the 1990s when P deposition increased as a result of atmospheric loading of dust transported from the Asian continent. The continued increase of anthropogenically produced dust may therefore impart damaging impacts on mountain ecosystems even if they are protected from direct anthropogenic disturbances.

DOI： 10.1007/s11284-012-0984-y

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We present a mathematical model of the maintenance of polymorphism of fluvial and amphidromous types in a fish population. Downstream drifting of offspring after hatching and their subsequent upstream migration have an important effect on the maintenance of an upstream population. This model, which assumes a linear relationship of offspring mortality rate, nutritional conditions, and flow rate or inclination with the distance from the river mouth, enables us to calculate the number of recruits to the mother population. From the present analysis, we can predict that the fluvial type producing large embryos is selected in the upper reaches under conditions where there are large differences in offspring mortality and flow rate between the upper and lower reaches, and a small difference in nutritional conditions between these sites. Therefore, the fluvial type can be more easily evolved in tropical regions than temperate regions where the difference in nutritional conditions is relatively larger. This selection is promoted when offspring mortality decreases greatly with their size. Conversely, at the lower reaches of the same river, the amphidromous type producing many small embryos is favored. These two populations can be the polymorphism of a single population species in a river, and can be established as different species in terms of pre- or post-reproductive isolation mechanisms, such as assortative mating or habitat preference, even if their members mate with each other in a river. We can assert that sympatric speciation is common in fluvial environments, when this process really works, because other taxonomic groups, such as Annelida. Crustacea, and Mollusca, also have the same sympatric pair of fluvial and amphidromous species derived from marine origin, that is, so called the evolutionary invasion from marine to terrestrial environments. (C) 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.ecolmodel.2012.06.006

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Glacial-interglacial scale climate variability in the tropical Pacific is often discussed with reference to modern El Nino-Southern Oscillation (ENSO). However, proxy records show diverse results and result in inconsistent interpretations. To investigate changes in the thermocline depth of the western Pacific warm pool (WPWP), which is strongly related to the strength of the Walker circulation, this study conducted delta O-18 and Mg/Ca analyses of multispecies planktonic foraminifera for the last 25,000 years. Habitat depths and calcification temperatures of seven foraminiferal species were estimated by comparing measured delta O-18 values with the predicted values by assuming isotopic equilibrium. Reconstructed Mg/Ca-based temperatures, calculated with a newly calibrated multispecies equation, showed similar to 1-2 degrees C larger cooling at the thermocline depth than that at the surface during the last glacial maximum (LGM), suggesting shoaling of the thermocline. Furthermore, fresher surface water in the WPWP, which is attributed to the suppressed evaporation by low sea surface temperature, suggests the weakened deep atmospheric convection. These results suggest that the Walker circulation during the LGM was weaker than that during the late Holocene. (C) 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.palaeo.2012.06.002

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：GEOCHEMICAL SOC JAPAN  

An apparent temporal increase in Mn and As in a thin surface layer (ca. 2 cm) of sediments in Lake Biwa, Japan, was observed by comparing depth profile data from 1977 and 2009. By our estimation, several thousand tonnes of Mn and a few hundred tonnes of As newly accumulated during the 32 years to 2009. Given various methodological concerns and geochemical factors associated with Mn and As dynamics, this phenomenon is believed to have been caused by increased Mn and As influxes to the lake rather than by redistribution induced by the intra-annual decrease in the level of dissolved oxygen. Mass balance calculations indicated that it is unlikely that the recently accumulated quantities of Mn and As were supplied via rivers as dissolved phases; rather, ground-water is the more likely source of these quantities. Further monitoring surveys will be important for assessing the environmental impact of this temporal change.

DOI： 10.2343/geochemj.2.0232

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1007/s11284-012-0984-y

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

This study determined the factors contributing to the spatial distribution of 14 metal concentrations in the surface sediments of Beppu Bay on the basis of comparisons of the organic geochemical properties and environmental parameters through principal component analysis (PCA) and redundancy analysis (RDA). The results of PCA and RDA showed that the concentrations of V. Cr, Co, and As were closely related to the distances between the sampling sites and the Oita River. This indicated that these metals originated from the river&apos;s drainage area. The Mn, Cu, Mo, and Cd concentrations were related to the water depth. These results indicated that the Mo. Cd, and Cu deposition processes were controlled by oxygen depletion, and that these elements accumulated in the deeper parts of the bay under anoxic conditions. (C) 2011 Elsevier Ltd. All rights reserved,

DOI： 10.1016/j.marenvres.2011.01.009

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Variations in sea surface temperature (SST) and vertical thermal structure in the western subtropical North Pacific, which has the largest air-sea heat flux of the world&apos;s oceans, provide insights into the mechanisms of climate change related to air-sea interactions. Here, we present planktonic delta(18)O and Mg/Ca records from the western subtropical gyre of the North Pacific spanning the last 30 kyrs. The results indicate that subtropical SSTs were approximately 3 degrees C lower during the last glacial than in the Holocene interglacial, indicating that glacial cooling occurred uniformly in the low to mid-latitudes of the western North Pacific. A decrease in intermediate depth temperatures at the late glacial suggests that the formation and/or advection of the subtropical mode water was enhanced due to a strong East Asian winter monsoon. The results suggest that the change in the thermal structure of the subtropical gyre was related to changes in East Asian monsoon activity. Citation: Sagawa, T., Y. Yokoyama, M. Ikehara, and M. Kuwae (2011), Vertical thermal structure history in the western subtropical North Pacific since the Last Glacial Maximum, Geophys. Res. Lett., 38, L00F02, doi: 10.1029/2010GL045827.

DOI： 10.1029/2010GL045827

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Language：Japanese   Publishing type：Research paper (scientific journal)  

We determined biogenic silica (BSi) content and estimated the Si budget within surface sediments (0-10 mm) of the eastern Seto Inland Sea, Japan, together with total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP). The BSi content determined by the DeMaster's technique ranged between 2.02 and 24.8 mg Si g^<-1>, with a mean (±S. D.) value of 11.6 ± 7.4 mg Si g^<-1> (n=25). The horizontal distribution of BSi in the area was related to that of TOC, TN and TP, and significantly correlated with the water content in the sediment (r=0.875). This indicates that the level of BSi content is influenced by the sediment characteristics, and the horizontal distribution of BSi reflects the re-distribution of fine particles as a result of hydrodynamic processes, as well as the amount of the phytoplankton in the overlying water column. The deposition rate of BSi, TOC, TN and TP from the water column to the surface sediment through biological activities in Osaka Bay-Harima Nada Area, a major part of the eastern Seto Inland Sea, is estimated to be 10.3×10^4 t Si yr^<-1>, 11.7×10^4 t C yr^<-1>, 1.41×10^4 t N yr^<-1> and 0.26×10^4 t P yr^<-1>, respectively. It was also found that the accumulation rate of BSi, TOC, TN and TP in the surface sediments accounts for 28%, 7.2%, 4.9% and 6.7% of Si, C, N and P production by phytoplankton in the same area, respectively. These results suggest that most particulate carbon, nitrogen and phosphorus produced by phytoplankton in the eastern Seto Inland Sea is transported to the outer area through water exchange whereas, in the case of particulate silica, sediment acts as a major sink.

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Other Link： http://dl.ndl.go.jp/info:ndljp/pid/10578318

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

We examined algal remains and fossil pigments in (210)Pb-dated sediment cores from Lake Biwa to explore historical changes in the phytoplankton community of the lake over the past 100 years and to identify environmental factors that caused those changes. Fluxes of fossil pigments and algal remains were very low before the 1960s, but increased through the 1960s and 1970s, indicating that the lake had eutrophied in the 20 years since 1960. After 1980, however, fluxes of all fossil pigments and algal remains decreased or stabilized. Redundancy analysis with meteorological and limnological variables explained more than 70% of the variation of these fluxes and showed that the decrease in fluxes of most algal taxa that occurred in the 1980s was related to changes in meteorological variables such as wind velocity, rather than changes in the lake's trophic state. Sedimentary records of algal remains also revealed that Aulacoseira nipponica, an endemic diatom species that grows in winter, decreased dramatically after 1980, while Fragilaria crotonensis, a cosmopolitan spring diatom species, became dominant. Replacement of one dominant diatom species by another could not be explained simply by changes in the lake trophic state, but was reasonably strongly related with an increase in winter water temperature. These results suggest that the phytoplankton community in Lake Biwa was influenced by changes in local environmental conditions (nutrient loading) through the 1960s and 1970s, but more so by regional (meteorological) and global (climate warming) factors since 1980.

DOI： 10.1007/s10933-009-9386-8

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Language：Japanese  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SAGE PUBLICATIONS LTD  

To assess abnormal warming of temperature in southwest Japan&apos;s coastal seas during the twentieth century, we developed a 200 yr interannually resolved record of planktonic and benthic foraminiferal Mg/Ca-based temperature using neritic coastal sediment cores. The winter to late spring bottom temperature (50 m) record, based on benthic foraminiferal (Nonion japonicum) Mg/Ca ratios in the Bungo Channel (BC), showed consistent variation with observed temperatures on a five-year average basis. The BC bottom temperature record showed a significant increasing trend of 1.5 degrees C/100 yr during the twentieth century, which was never apparent in nineteenth century, That result suggests that our Mg/Ca-based thermometry approach using coastal benthic foraminifera can detect abnormally rising temperatures in neritic coastal seas in southwest Japan. The abnormal warming of winter to late spring bottom temperature in the BC contrasts with the lack of an increasing trend in the Globigerinoides ruber-based summer to autumn temperature for the upper 20 m in the slope region of southwest Japan and the lack of an increasing trend in the steric sea level in the region. These results indicate a warming trend of the neritic coastal ocean in southwest Japan, especially in winter to late spring. The timing of the onset of the prolonged abnormal high-temperature stage, which started in the early twentieth century, suggests a link of the neritic coastal ocean in Japan with human-induced global warming.

DOI： 10.1177/0959683608100573

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS LTD ELSEVIER SCIENCE LTD  

Pinctada fucata martensii mantle tissue and gut contents were examined as baseline indicators of carbon and nitrogen isotope composition at six stations in the Uwa Sea, Japan. Substantial variations in delta C-13 and delta N-15 values of oysters among stations were observed, with delta C-13 being consistently lower at Hiburi Island (-18.1 parts per thousand) than at other stations (-17.2 parts per thousand). Oysters from fish farm sites were enriched in delta N-15 (8.1 parts per thousand) relative to those from unaffected sites (6.8 parts per thousand), suggesting that fish farming tends to increase baseline delta N-15 values. The mean Delta delta C-13 (0.8 parts per thousand) was consistent over space and time, whereas the average Delta delta N-15 slightly increased in summer. The relatively low delta N-15 enrichment compared to the theoretical isotope fractionation factor (3.4 parts per thousand) may be due to oyster-specific physiological attributes. Carbon and nitrogen isotope turnover rates were roughly similar within a tissue, and mantle tissue turnover rate was estimated to be 120-180 days. These results indicated that oysters are long-term integrators of delta C-13 and delta N-15 from their diet and that delta C-13 of oysters is a more accurate bioinclicator of isotopic baselines than delta N-15 for marine ecological studies. (C) 2008 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ecss.2008.03.004

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS LTD- ELSEVIER SCIENCE LTD  

We estimated the composition of two food sources for the cultured pearl oyster Pinctada fucata martensii using stable isotopes and stomach content analysis in the coastal areas of the Uwa Sea, Japan. The delta C-13 values of oysters (-17.5 to -16.8 parts per thousand) were intermediate between that of particulate organic matter (POM, -20.2 to -19.1 parts per thousand,) and attached microalgae on pearl cages (-13.0 parts per thousand). An isotope mixing model suggested that oysters were consuming 78% POM (mainly phytoplankton) and 22% attached microalgae. The attached microalgal composition of the stomach content showed a strong resemblance to the composition of that estimated through the isotope mixing model, suggesting preferential utilization of specific components is unlikely in this species. These results indicate that P. fucata martensii feed on a mixture of phytoplankton and attached microalgae, and that the attached microalgae on pearl cages can serve as an important additional food source. (C) 2007 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ecss.2007.07.046

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS LTD ELSEVIER SCIENCE LTD  

In (order to detect responses of primary productivity in Japanese coastal embayments to climate and watershed changes for the last 500 years, we unraveled sedimentary records of delta C-13, delta N-15, TOC, TN, and C/N ratio in the north basin of Kagoshima Bay (KB). Based on principal component analysis of these geochemical data, primary component (PC) I that explains 65% of the total variance within all the geochemical parameters was identified. The records of delta C-13, delta N-15, TOC, and TN having high loadings on the PCI axis showed centennial-scale variations (low levels during AD 1595-1725 and high levels during AD 1725-1860) and a shift (AD 1725). A comparison between our records and previous studies on the biogeochemical processes suggests that the factor responsible for fluctuations in delta C-13, delta N-15, TOC, and TN is likely to be the changes in primary productivity in the north basin of KB rather than other factors. C/N values, which have high loadings on PC2, are possibly related to input of C-3 land plants to the north basin of KB, suggesting changes in the surrounding forest environments. The centennial-scale decrease in primary productivity that is represented by the TOC record is coincident with a temperature decrease associated with the Little Ice Age, suggesting that the primary productivity in the north basin of KB might have been influenced by global or Northern Hemispheric-scale climate changes. (C) 2007 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ecss.2007.01.010

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS LTD ELSEVIER SCIENCE LTD  

In order to elucidate the organic and sulfur geochemical processes of surface sediments associated with oxic/anoxic conditions, we have examined the relationship between the spatial distribution of sedimentary delta C-13, delta N-15, TOC, TN, TS, C/N ratio, and C/S ratio and environmental variables in Beppu Bay, Japan.
The results of the principal component analysis and redundancy analysis of these geochemical parameters revealed two dominant patterns in their variations. The first pattern is characterized by variations in delta C-13, delta N-15, and C/N, which are closely related to the distance between the river mouth and sampling sites. This pattern is attributed to the mixing of organic materials from two sources-C-3 land plants and marine phytoplankton-rather than various alternation processes such as diagenesis and eutrophication. The second pattern is characterized by variations in TOC, TN, TS, and C/S. The RDA results showed that these geochemical parameters are closely related to the water depth and DO. Meanwhile, a simple correlation analysis also showed a good correlation of TOC, TN, TS, and C/S with bottom temperature and Eh. In Beppu Bay, a significant negative correlation is observed between the depth and the dissolved oxygen concentration and Eh, indicating that the deeper bottom environments have lower oxygen availability and are more reductive. According to these observations and some findings of previous studies, the spatial variations in TOC and TN are controlled by decomposition rates. This is in contrast to those in TS and C/S, by rate of pyrite formation. Both these rates are possibly influenced by temperature or oxic/anoxic conditions.
The RDA results revealed that the two patterns in the variations of the geochemical parameters exhibit a perpendicular relation with respect to RDA axes. This indicates that the two geochemical groups are controlled only by their main factors. It is possible that TOC, TN, TS, and C/S are sensitive to temperature or oxic/anoxic conditions in Beppu Bay; in contrast, delta C-13, delta N-15, and C/N are less sensitive to these conditions. (c) 2006 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ecss.2006.11.005

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japanese Society of Diatomology  

In order to explore the origin and evolution of an endemic diatom species, <I>Stephanodiscus suzukii</I> Tuji & Kociolek, living in Lake Biwa, species and morphological changes of <I>Stephanodiscus</I> spp. for the last 400 ka were elucidated using samples from a 140 m-long core. The first appearance of S. suzukii like-species was approximately 390 ka, close to the timing of the onset of Lake Biwa basin formation. However, their dominance started at about 170 ka. Since their first appearance, changes in dominance from S. sp. D, to <I>S. suzukii</I> can be recognized. During periods of transition, transitional forms of the two species were present in the populations. Morphological changes are apparent in the location of interrupted areolae fascicles, the number of areolae per fascicle and number of fascicles per 10 lam near the margin. The existence of individuals with transitional forms and slow morphological changes during the transition period suggest that S. sp. D, and <I>S. suzukii</I> are identical species. The changes in morphology of S. suzukii are related to shifts in global climates and local crustal movements, suggesting they are survival strategies against severe climate and limnetic changes.

DOI： 10.11464/diatom1985.23.0_71

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

In order to examine the responses of primary productivity in the southern coastal sea of Japan to the Pacific Decadal Oscillation (PDO) in the 20th century, sedimentary records of diatom productivity (diatom valve fluxes) were reconstructed using core samples from the Bungo Channel (BC) in southwest Japan. The record of the Thalassionema spp. flux-the best index of fall primary productivity in the BC-indicated a multidecadal-scale duration with a low flux (1943-1982) and those with a. high flux (1913-1943 and 1982-2001); apparent shifts were recognized in 1943 and 1982. The shift in 1982 was also recognized in the flux records of other early summer to fall predominant genera in the BC and, previously, in the biogenic silica records from a broad region of the southeast BC. This indicates that in our records, this shift reflects a general trend in the primary production in the southeast BC. A comparison among the Thalassionema spp. flux records, meteorological data from an observatory adjacent to the core site, and the PDO index showed that the flux records were more similar to the PDO index than the other meteorological records, which suggests that the multidecadal-scale variability of the BC primary productivity may be associated with some marine-derived forcing. The bottom intrusions of nutrient-rich water that upwelled from the shelf slope into the BC, the axis movement or the transport of the Kuroshio Current off the BC, and a basin-scale wind stress in the North Pacific might play an important role in this forcing and mediate between the BC primary productivity and the PDO.

DOI： 10.1007/s10872-006-0084-0

Web of Science

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：KLUWER ACADEMIC PUBL  

To reconstruct the pattern of past climate change in central Japan during the last 140 kyr, total planktonic diatom valve concentrations (valves g(-1)) and fluxes (valves cm(-2) year(-1)) of total planktonic diatoms flux (PVF) and individual species were examined using a 140-m core taken from Lake Biwa, Shiga Prefecture. Most records had a sample resolution between approximately 150 and 300 yr. Based on characteristics of past and modern diatom responses to possible climate variables, we interpreted changes in Stephanodiscus suzukii flux (SVF) to reflect changes in phosphorus levels, which reflect, in turn, summer precipitation levels; changes in Aulacoseiva nipponica flux (AVF) reflect winter vertical lake-water mixing induced by winter temperatures and snowfall levels. Thus, changes in total planktonic diatom flux reflect a combination of summer precipitation, winter temperature, and snowfall values. During the 140-101 ka interval, changes in S. suzukii productivity at a millennial timescale may correspond to changes in summer rainfall in central Japan. The disappearance of A. nipponica during the same period could indicate weaker vertical mixing, possibly caused by increased temperatures and decreased snowfall levels in winter. During the 101-70 ka interval, the AVF record shows levels near or above those observed in present times, indicating that winter water temperatures fell within the optimal range for A. nipponica to prosper. Generally low AVF values during the 70-7 ka interval indicate weak winter vertical mixing and cold winters. The many intervals with low PVF values during the same period suggest decreased summer precipitation levels. Between 7 and 0 ka, PVF, SVF, and AVF records show levels near or above those of the present, suggesting winter temperatures favorable for A. nipponica growth, and snowfall and summer precipitation levels probably similar to or above those currently recorded.

DOI： 10.1023/B:JOPL.0000025284.96452.26

Web of Science

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Association for Quaternary Research  

For high-resolution reconstruction of climate change in Lake Biwa catchment area, we have analyzed concentration of diatom valve (DVC; valves g^-1) and diatom valve flux (DVF; valves cm^-2 yr^-1) for the past 140ka using the upper part of a 140-m core from Takashima-oki. Time resolution of sampling intervals is 150 to 500 years.<br>The DVF records show higher values during the Holocene (0-7ka) and the last interglacial age (68-135ka) than the present value. By contrast, during the last glacial age (7-68ka) the DVF is relatively low. In addition, the DVF record shows millennial-scale fluctuations over the last 140ka.<br>We have checked the relationship among the DVF, sedimentation rate, and climatic parameters such as temperature, snow depth, and precipitation. We concluded that the DVF reflects diatom productivity, which is strongly controlled by the precipitation level of the preceding summer period.<br>If the DVF can be a proxy for precipitation, the characteristics of the precipitation around Lake Biwa are summarized as follow: the low-level duration during 56-68ka, corresponding to the oxygen-isotope stage 4, is longer than that during the LGM (oxygen-isotope stage 2); after 8ka, the levels gradually rise toward 5.5ka, and increased levels occur at 3 and 5.5ka.<br>The pattern and the episodic events in the record of the DVF in the Lake Biwa sediment can be very closely correlated to the Dansgaard-Oeschger episodes in the North Atlantic climatic record which is revealed from the Greenland ice core. This concordance indicates a possible teleconnection between the precipitation over Japan and temperature over the North Atlantic. The possible mechanism of the teleconnection might be related to changes in westerlies which are driven by temperature changes over the North Atlantic.

DOI： 10.4116/jaqua.42.305

CiNii Books

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Other Link： https://jlc.jst.go.jp/DN/JALC/00232466667?from=CiNii

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Detailed diatom records from a 140-m sediment core with a time resolution of approximately 1000-1500 yr were obtained from Lake Biwa to examine possible correlations between diatom concentrations and paleoclimatic changes during the last approximately 400 ka. In the sediments, the diatom assemblages are generally dominated by planktonic diatoms. The diatom stratigraphy is characterized by four zones with distinctly higher concentration values and 12 horizons with low concentration values. Diatom records from the 140-m core and another core indicate that these changes were not local phenomena, but occurred throughout the lake basin. Comparison of diatom concentration records with pollen records from Lake Biwa sediments indicates that higher diatom concentrations represent warmer and wetter climatic conditions. and that lower concentrations represent colder and drier climatic conditions. Comparison of diatom records from Lake Biwa with marine oxygen isotopic records shows that the number of peaks and their relative amplitudes are similar in both profiles; therefore, peaks and drops in diatom concentration profile might correlate with most oxygen isotopic events. This finding indicates that during approximately the last 400 ka, the changes in diatom concentration, represented as diatom productivity in Lake Biwa, correlate closely with global paleoclimatic changes at the scale of the Milankovitch cycle (precession cycles of 23 and 19 ka), and that diatom concentration was higher during warm events and lower during cold or cooling events. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0031-0182(01)00490-4

Web of Science

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Geological Society of Japan  

Diatoms were examined from a 13-m-long sediment core from the Misaka peatland, Gifu Prefecture, covering the late glacial through the Holocene (ca. 0-30 ka). Diatom records produce a history of paleoenvironmental changes as follows. During most of period between ca. 30 and ca. 12 ka of the late glacial, the lake was eutrophic and neutral or alkaline with an open water area. However, during two short periods (2.5-2.4,1.6-1.5 ka) of the late glacial, the lake became oligotrophic and acid as bog pool or low moor, probably with enhanced hygrophytes. During the open water lake intervals water temperature was relatively lower. On the contrary, during the bog pool or low moor intervals, it was relatively higher. These environmental changes depend on air temperature fluctuations for the late glacial. Between ca. 12 and ca. 4 ka, the lake became oligotrophic and acid as bog pool or low moor with enhanced hygrophytes. Diatom assemblages during this interval indicate high water temperature, probably in response to Holocene warmth. The present study shows close correlation between paleoclimate change and a environmental history of the Misaka peatland.

DOI： 10.5575/geosoc.107.451

CiNii Books

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Other Link： https://jlc.jst.go.jp/DN/JALC/00382678255?from=CiNii

Publisher：GEOCHEMICAL SOCIETY OF JAPAN  

DOI： 10.14862/geochemproc.58.0.115.0

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Publisher：GEOCHEMICAL SOCIETY OF JAPAN  

DOI： 10.14862/geochemproc.58.0.112.0

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Publisher：GEOCHEMICAL SOCIETY OF JAPAN  

DOI： 10.14862/geochemproc.55.0.413.0

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Language：Japanese   Publisher：The Geological Society of Japan  

DOI： 10.14863/geosocabst.2006.0_277_1

CiNii Books

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DOI： 10.4116/jaqua.36.113

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